1. Field of the Invention
The present invention relates to an inverter apparatus for supply of electric power to a load which has to be controlled over a wide range of currents, and more specifically, to an inverter apparatus appropriate for use with a light source employing a dimness adjustable cold cathode tube.
2. Description of the Prior Art
Inverter apparati are provided for converting a direct current of electric power to an alternating current form and are also known as an inverse converters for use in a variety of electric systems.
FIG. 6 shows a circuit diagram of a conventional inverter apparatus specified for a discharge tube. As shown in FIG. 6, a booster transformer 10 in a Royer oscillator circuit comprises a primary coil 10P, a secondary coil 10S, and a feedback coil 10F. The Royer oscillator circuit includes NPN switching transistors 11 and 12 as well as the booster transformer 10. There are also provided a capacitor 13 for voltage resonance and a choke coil 14. Accordingly, when the transistors 11 and 12 are not conductive, their voltage outputs have a sine-waveform and the waveforms of voltages at the primary coil 10P and secondary coil 10S of the transformer 10 are also sine-waves. The choke coil 14 is connected at input to a DC/DC converter described later and at output to a cold cathode tube 31.
As the inverter performs self-oscillation, its output delivers a high voltage of sine-wave having a frequency of some tens KHz and hence, the cold cathode tube 13 is illuminated. Also, an integrated circuit (IC) 20 is provided which serves as a step-down chopper for controlling the base of a PNP switching transistor 21 which is a component of the DC/DC converter. The IC 20 comprises an oscillator OSC for producing a triangle wave, two operational amplifiers A1 and A2 for comparing action, a PWM comparator COMP for comparing between output of the oscillator OSC and output of one of the two operational amplifiers A1 and A2, and an output transistor 113 driven by the PWM comparator COMP for conducting the base of the PNP switching transistor 21. The PWM comparator COMP of the IC 20 has two comparing inputs, one connected to the oscillator OSC and the other to the two operational amplifiers A1 and A2. The output of the oscillator OSC is compared with a higher one of the two voltage outputs of their respective operational amplifiers A1 and A2. The IC 20 will now be referred to as a DC/DC converter controlling IC for ease of the description so long as its arrangement is not modified but may be used for other applications. Further shown are a fly-wheel diode 22, a choke coil 23, and a capacitor 24 which constitute in combination an LC filter. Denoted by 25 and 26 are a capacitor and a resistor respectively for determining the frequency of oscillation. There are C and R elements 27 to 30 for adjusting the paired inputs of the operational amplifiers A1 and A2 in phase with each other in the IC 20. Two diodes D15 and D16 are connected for rectifying positive components of a discharge current across the cold cathode tube 31. Elements 18 and 19 are a resistor and a capacitor, respectively, which form a lowpass filter for shaping the current waveform. The output of the lowpass filter is connected to a positive input of the operational amplifier A2 in the DC/DC converter controlling IC 20. In action, a voltage which is proportional to an average of positive cycles of the discharge current appears across the capacitor 19.
The voltage is then compared by the operational amplifier A2 with a reference voltage of the DC/DC converter controlling IC 20. A resultant voltage output is proportional to a difference between the two voltages. The resultant voltage output is fed to the PWM comparator COMP where it is compared with the triangle waveform of the oscillator OSC in the DC/DC converter controlling IC 20, as shown in FIG. 7. If the discharge current is increased by any incident, the voltage output of the operational amplifier A2 is shifted from the line B to the line A. This causes the output of the PWM comparator to shift from the line C to the line D in FIG. 7. Accordingly, the on-time of the PNP switching transistor 21 is decreased attenuating the voltage output of the DC/DC converter and thus the source voltage of the Royer oscillator circuit. As the result, the discharge current is decreased. In other words, the discharge current can be controlled to a constant rate. Denoted by 32 and 33 are resistors for adjusting the voltage output of the DC/DC converter to a constant level. If the cold cathode tube 31 is not installed, the resistors 32 and 33 detect and control the voltage output of the DC/DC converter for setting the voltage across the secondary coil 10S of the booster transformer 10 to a constant level before starting a discharge action. The joint between the two resistors 32 and 33 is connected to the positive input of the operational amplifier A1 in the DC/DC converter controlling IC 20, thus forming a negative feedback loop and allowing a constant voltage output from the DC/DC converter. Both outputs of the operational amplifiers A1 and A2 are OR connected so that a higher level of the two outputs of the amplifiers A1 and A2 is selectively transferred to the PWM comparator COMP.
It is known to use a semi-class E voltage resonance inverter for increasing the operational efficiency. The semi-class E voltage resonance inverter is however not favorable to operate zero-voltage switching of its power switch in response to a variable change in the input voltage. In the non-zero voltage switching mode, it may produce not negligible levels of switching noise and loss. It is thus an object of the present invention to provide a voltage resonance inverter circuit capable of performing a zero-voltage switching action whenever a change in the input voltage occurs.